1. Field of the Invention
The invention generally relates to the removal of elemental mercury. In particular, the invention provides methods for the removal of elemental mercury from a gas, typically air, via selective photoionization, followed by electrostatic precipitation.
2. Background Description
The minimization of mercury emissions from coal-fired power plants is of great interest since mercury is a potential human health hazard, even at very low concentrations. A very recent EPA notice of regulatory finding1 states (on page 79,827, in the first paragraph, first sentence), xe2x80x9cBased on the assessment of hazards and risks due to emissions of HAP (Hazardous Air Pollutants) from electric utility steam generating units, mercury is the HAP of greatest concern.xe2x80x9d Coal-fired utilities are currently the source of approximately one third of the mercury deposited across the U.S.2 Much of this mercury is in elemental form, a form that can remain in the atmosphere for long periods of time2. Some of this mercury lands in lakes where it is readily converted into methylmercury, a bioaccumulating neurotoxin.1,2 
The EPA notice of regulatory finding1 goes on to explain that mercury in the flue gas from coal combustion may be present in three different forms. The forms, called species, include elemental mercury, divalent oxidized forms, and mercury adsorbed onto the surface of fly ash or other particles. Adsorbed mercury onto particles is removable using conventional devices such as electrostatic precipitators (ESP). The divalent forms of mercury are generally water soluble and removable in wet scrubbers or in flue gas desulfurization (FGD) systems1,3. However, xe2x80x9celemental mercury is insoluble in water, does not react with alkaline reagents used in FGD systems, and cannot be captured in wet scrubbers,xe2x80x9d1 consequently elemental mercury remains mostly unremoved from flue gas.
Even when equipped with a dry ESP and a FGD, coal-fired utilities fail to remove about a third of the mercury from burnt coal3. Almost all of this mercury is in elemental form. In a dry ESP much of the elemental mercury either does not become ionized or is re-entrained in the gas during the rapping cycle of the ESP4. Since mercury is not soluble in water and does not react with alkaline agents, FGD systems and wet scrubbers also fail to remove the elemental mercury. Even when fully equipped, the atomic mercury concentration2,3 in a typical coal-fired utility""s exhaust is 1 to 10 xcexcg per cubic meter (90 to 900 parts per trillion).
It would be of great benefit to have available methodology designed to remove elemental mercury from such sources and to preclude its deposition into the environment.
It is an object of the invention to provide a method and apparatus for removing mercury from exhaust effluents.
According to the invention, an exhaust from a coal fired furnace, incinerator, chlorine plant or the like, which is likely to contain mercury, is passed through a first electrostatic precipitator or other suitable device to remove particulate matter. Mercury in the exhaust is exposed to light of wavelength which raises the mercury to an excited state. Light of 253.65 nm is suitable for this purpose. Subsequently, the excited mercury is ionized. This is preferably accomplished using light of a different wavelength. A second electrostatic precipitator removes the ionized mercury from the exhaust so that exhaust emitted to the environment from the exhaust stack is substantially reduced in mercury concentration (e.g., preferably greater than 90% reduced). The process is enhanced by nucleating water particles onto the ionized mercury, such as by exposing the exhaust to supersaturated water vapor. This allows for the electrostatic precipitation of charged water droplets. Sulfur oxides (e.g., sulfur oxide and sulfur dioxide) may also be removed from the exhaust using a wet scrubber or similar apparatus.